Acid paint remover

ABSTRACT

Disclosed is an aqueous acidic developing solution for a photoresist comprising an acid having from about 1 to 8 carbon atoms, in an effective combination with a solubilizing solvent for the photoresist.

FIELD OF THE INVENTION

The present invention relates to developing solutions. Morespecifically, the present invention relates to aqueous acidic developingsolutions for negative photoresists.

A BRIEF DESCRIPTION OF THE PRIOR ART

Aqueous acidic developing solutions are generally known in the art. Thedeveloping compositions are used to selectively remove solubilizableportions of photoresists from the insolubilized portions thereof whichhad been exposed to patterned irradiation. Unfortunately, many of theprior art aqueous acidic solutions are not selective enough to speedilyremove the solubilizable portions without removing the insolubilizedportions of the photoresists. In an attempt to make the developingsolutions more selective, the art often resorts to diluting the aciddeveloping solutions. However, the dilute solutions tend to be lessefficient in removing the solubilizable portions of the photoresist.Thus the efficiency of the solution as a developer becomes diminished.

By the present invention, there is provided an aqueous acid developingsolution that speedily develops a negative photoresist without removingthe insolubilized portions of the photoresist.

SUMMARY OF THE INVENTION

In accordance with the foregoing, the present invention encompasses anaqueous acidic developing solution for a photoresist comprising an acidhaving from about 1 to 8, preferably 1 to 3 carbon atoms, in aneffective combination with a solubilizing solvent for the photoresist.In a preferred embodiment of the invention, the aqueous acidic solutioncomprises a mixture of acids containing glycolic acid as a first acidand lactic acid as a second acid, in combination with a photoresistsolubilizing solvent comprising ethylene glycol monobutyl ether. Anaqueous acidic solution comprising the combination of the mixture ofacids and the photoresist solubilizing solvent, at a concentration ofabout 1% to 5% by weight of the combination of the mixture of acids andthe photoresist solubilizing-solvent based on total weight of aqueousdeveloping solution, has been found to be a good developing solution ata temperature of about 80° to 120° F.

Also encompassed by the present invention is a process for developing anegative photoresist comprising contacting the photoresist with thedeveloping solution of the present invention for a period of timesufficient to remove the solubilizable portion of the photoresist whileleaving the irradiated photoresist intact.

By the term "photoresist solubilizing solvent" is meant that thestructural conformation of the solvent is such that it is capable ofcontacting and preferably penetrating the photoresist to some degree atthe solid liquid interface. This penetration of the photoresist disruptsits molecular parking and changes the density or softens thephotoresist. The softened photoresist is now more susceptible to removalby dispersion in an immersion bath of developer solution or by kineticmethods such as spraying developer solution. By the term "effectivecombination" is meant that when using an acid and solvent developingsolution one must establish a balance in respect to the variableefficacies of solvents with the variable efficacies of different acidsuseful in the invention. For example, if a very effective solventdescribed as being relatively low in molecular weight and linear inconformation without obstructive side groups is used in concert with avery effective acid described as being relatively low in molecularweight with a high ionization potential, then both the solubilizable andinsolubilized portions of the photoresist will be indiscriminatelyremoved. Likewise if large and bulky solvents and acids are used, therate of attack on the solubilizable photoresist will be too slow to beuseful. The key to an "effective combination" is to temper or offset thevariable strengths or weaknesses of the solvents with those of theacids. This balance provides a developing solution with a rate of attackthat distinguishes the structural and chemical differences betweensolubilizable and irradiated photoresist.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, the principal ingredients of theaqueous developing solution are an acid having from about 1 to 8,preferably 1 to 3 carbon atoms, and a photoresist solubilizing solvent,which are present in an aqueous medium. It is believed without beingbound that the lower the carbon content of the acid, the faster is itsrate of removing the photoresist. Similarly, it is believed that thelower the molecular weight of the photoresist solubilizing solvent, thefaster is its rate of removal of the photoresist. The converse of theforegoing is believed to be true. It has, however, been found that inrelative terms and with respect to the rate of removal of thephotoresist, a combination of a fast-acting acid or an acid with a highefficacy and a slow solubilizing solvent with a low efficacy provides amore efficacious removal of the photoresist. In any event, the rate ofremoval of the photoresist would depend as well on the temperature ofthe developing solution and the pressure at which it is applied.

In accordance with this invention, it has been realized that the effectof a developing solution is characterized by the molecular weight of theacid and further characterized by the molecular weight of thesolubilizing solvent, and the polarity and/or configuration of thesolubilizing solvent. Illustratively, given a useful solubilizingsolvent, it is believed that the rate of removal of the acids increaseswith the decrease in the carbon content of the acid. Conversely, given auseful acid in accordance with the invention, it is believed that therate of removal of the photoresist increases with the decrease inmolecular weight and/or polarity of the solvent. It has been determinedin accordance with this invention that the developing solution comprisesa specified acid containing from about 1 to 3 carbon atoms which byitself can solubilize the photoresist, in combination with asolubilizing solvent which by itself is almost totally ineffective insolubilizing the photoresist. Illustratively, an acid such as aceticacid which by itself can solubilize the photoresist can be employed incombination with a solubilizing solvent such as propylene glycol whichis by itself totally ineffective in solubilizing the photoresist.

In the present embodiment, the invention comprises a water-solublemixture of at least two acids of low molecular weight in combinationwith a photoresist solubilizing solvent. The first acid contains fromabout 1 to 8 carbon atoms and preferably from about 1 to 3 carbon atoms.The first acid is present in an amount of about 15% to 35%, andpreferably 23% by 28% by weight based on weight of acids andsolubilizing solvent. Illustrative but non-limiting examples of thefirst acid can be formic acid, acetic acid, hydroxy acetic acid, oxalicacid and lactic acid. The second acid is present in an amount of about2% to 15%, and preferably 5% to 8% by weight based on weight of acidsand solubilizing solvent. The second acid contains from about 1 to 8carbon atoms and preferably from about 1 to 3 carbon atoms. Illustrativebut non-limiting examples of the second acid can be tactic acid,oxalicacetic acid, acetoacetic acid, propionic acid, succinic acid andbutyric acid.

The solubilizing solvent useful herein is selected from the groupconsisting of diethylene glycol monobutyl ether, ethylene glycolmonobutyl ether, propylene glycol methyl ether, dipropylene glycolmethyl ether, tripropylene glycol methyl ethyl ether, propylene glycolt-butyl ether, ethylene glycol methyl ether, ethylene glycol ethylether, triethylene glycol butyl ether, diethylene glycol methyl ether,triethylene glycol methyl ether, ethylene glycol propylether, hexyleneglycol, dipropylene glycol methyl ether acetate, propylene glycol methylether acetate, ethylene glycol monobutyl ether acetate and ethyleneglycol phenyl ether. The solubilizing solvent is present in an amount ofabout 50% to 83%, and preferably 63% by 70% by weight based on weight ofacids and solubilizing solvent.

In the preparation of the aqueous acidic solution of the invention, theingredients set forth hereinabove can be admixed with water in anyorder. Optionally, other ingredients such as acid stable surfactants canbe formulated into the developing solution. An example of thesurfactants that can be used herein include polyglycol resins,acetylenics and fluorosurfactants. The resultant aqueous developingsolution typically has an acid concentration of about 0.05% to 20% byweight, preferably 0.25% to 4% by weight, more preferably 0.5% to 1.5%by weight based on total weight of aqueous developing solution. Theconcentration of solubilizing solvent is 1% to 5%, preferably 1 % to2.5% by weight based on total weight of aqueous developing solution.

In the practice of this invention, the developing solution is used tocontact a photoresist so as to remove the solubilizable portionsthereof. The photoresist is contacted with the developing solution byspraying or by immersion means. Typically, the developing solution isemployed at a temperature of about 70° F. to 120° F. and preferably 80°F. to 900° F., over a period of about 30 seconds to 120 seconds andpreferably 30 seconds to 60 seconds. For a negative photoresist theunexposed portions thereof (during irradiation) are removed from thesubstrate with the developing solution. Thereafter, the substrate withthe insolubilized photoresist can be rinsed with water.

In accordance with the present invention, it has been determined thatthe presence of the second acid unexpectedly enhances the speed andextent of removing the solubilizable portion of the photoresist, withoutremoving the insolubilized portions thereof.

These and other aspects of the invention are further illustrated by thefollowing non-limiting examples. The first example shows the developingsolution and a method of preparing the same. The other examples show thephotoresists that can be developed in accordance with the invention andmethods of making and developing the same.

EXAMPLE 1

The aqueous developing solution of the invention and a method ofpreparing the same:

    ______________________________________                                        % w/w                                                                         ______________________________________                                        67.5       Butyl Cellosolve (a solubilizing solvent)                          25.8       Glycolic Acid (First Acid)                                         6.7        Lactic Acid (Second Acid)                                          ______________________________________                                    

3% by weight of the above concentrate was measured into water to producean aqueous developing solution of the invention, as follows.

    ______________________________________                                                       % w/w                                                          ______________________________________                                        Water            97.000                                                       Butyl Cellosolve 2.025                                                        Glycolic Acid    0.774                                                        Lactic Acid      0.201                                                                         100.000                                                      ______________________________________                                    

EXAMPLE 1A

The following illustrates the preparation of the photoresist and theprocess of developing the same. The photoresist of this instance was anelectrodepositable composition which was prepared as follows.

Isophorone diisocyanate "IPDI" (687.4 grams (g) having 6.21 NCOequivalents) were charged into a 3-liter four-neck round bottom flaskwhich was equipped with a mechanical stirrer, a thermometer, a droppingfunnel, and a condenser. A calcium sulfate drying tube was attached tothe condenser to protect it from moisture. The flask was heated to 50°C. under stirring. Dibutyltin dilaurate (0.4 g) was added to the IPDIand a mixture of 2-hydroxy-ethyl acrylate (367.8 g, 3.17 OH equivalents)and IONOL®, which is a di-t-butyl-p-cresol (6.3 g), was added to theIPDI at 50° C. to 55° C. through the dropping funnel over the period of1.5 hours. After the addition was completed, the mixture was stirred at500° C. for an additional 4 hours to give urethane acrylate (NCOequivalent weight of 350.8) in which free IPDI was estimated to be 0.2percent (wt/wt) present by HPLC (High Pressure Liquid Chromatography).

EXAMPLE 1B

A mixture of EPON® 1001 (an epoxy resin from Shell Chemical Co. havingepoxy equivalent weight "EEW"=534; 1068.0 g) and 4-methyl-2-pentanone(224.3 g) was charged into a 3-liter four-neck round bottom flask whichwas equipped with a mechanical stirrer, a thermometer, and a Dean-Starkcondenser. The flask was heated to reflux in order to remove moisturefrom the epoxy resin. After the removal of moisture, the flask wascooled to 65° C. and 2-(methylamino)ethanol (1.5 mole; 112.7 g) wasadded through a dropping funnel over a period of five minutes. Anexothermic reaction took place as soon as the addition was completed,and the reaction temperature was maintained at 110° C. until EEW becamegreater than 24,000. The flask containing the resultant reaction productcomprising an epoxy-amine adduct was then cooled to 80° C. and a mixtureof the urethane acrylate from Example 1A (338.2 g; 0.99 NCO equivalents)and 4-methyl-2-pentanone (300 g) was added to the amine-epoxy adduct at80° C. to 85° C. through another dropping funnel over a period of 3hours. The resulting mixture was stirred at 80° C. for an additional 2hours to ensure the consumption of the isocyanate groups. EBECRYL® 3600(an epoxy diacrylate available from Radcure Corporation, 519.9 g) wasadded to the warm amine-functional resin and the whole mixture was wellmixed at 75° C. in the flask. The blended resin mixture (2000 g, 1590.8g solids) was dispersed by addition to lactic acid (53.7 g, 0.527equivalents) in deionized water (838.7 g). The aqueous dispersion wasfurther reduced to 35.0 percent (wt/wt) by addition of deionized water(1652 g). 4-methyl-2-pentanone was azeotropically distilled underreduced pressure from the aqueous dispersion, giving an aqueousdispersion of amine-functional resin; solids: 39.1 percent;milliequivalent "MEQ". acid: 0.124; MEQ base: 0.304; particle size:1440,Å; (number average molecular weight) "Mn": 17,924; (weight averagemolecular weight) "Mw": 95,125; "Mz": 468,738 (Z average molecularweight).

EXAMPLE 2A

The procedure of Examples 1A and 1B was repeated except that 0.375 moles(25 mole percent) of the 2-(methylamino)ethanol were replaced withdibutyl amine (48.5 g). After dispersion in water and stripping of thevolatile solvents, a milky white dispersion resulted with the followingcharacteristics:

(1) Total solids at 110° C.--37.2 percent; (2) Milliequivalent acid pergram --0.147; (3) Milliequivalent base per gram--0.305; (4) Particlesize--2290,Å (measured by laser light scattering on Coulter N4 particlesize analyzer available from Coulter Company); and (5) Molecular weightMn=12,130, Mw=75,883, Mz=611,366.

EXAMPLE 2B

An electrophoretic bath was prepared from Example 2A by first reducingthe prepared resin to 30 percent solids using deionized water. Mildagitation was used to blend 1425.7 grams of this reduced polymerdispersion with 48.6 grams of trimethylol propane triacrylate. Asolution of 19.4 grams of 2-isopropyl thioxanthone and 19.4 grams of2-hydroxyethyl n-hexanol (hexyl CELLOSOLVE®) was then added to the resinblend under mild agitation. The entire bath was then reduced to 10percent solids using deionized water. The resulting paint bath had a pHof 4.53 and a conductivity of 750 micromhos/centimeter (cm).

Epoxy-fiberglass circuit boards (available from Nelco, Inc.) measuring 8cm 12×cm with 1/2 ounce per square foot of copper on one side wereplaced in a detergent solution (ASTM D 2248-65) at 165° F. for 5minutes. The panels were then washed with hot water, followed by adeionized water rinse and oven baked at 82° C. for 5 minutes. The panelswere submerged in the prepared electrophoretic bath along with astainless steel electrode measuring 5 cm×24 cm. Each panel was attachedto a direct current (DC) power supply, the panel being the cathode andthe stainless steel electrode the anode. The cathode and anode wereseparated by a distance of approximately 5 cm with the copper side ofthe board facing the stainless steel anode in the bath. The bathtemperature was held at 85° F. and a magnetic stirrer was used toprovide mild agitation. An electrical potential of 30 volts was appliedfor 30 seconds, after which time the panel was removed, rinsed with awater spray, followed by a 0.1 percent by weight SURFYNOL® TG (availablefrom Air Products Corporation)/deionized water rinse. The panel was thenflashed in a forced air oven at 82° C. for 60 seconds.

Assuming a coating weight of 30 grams per meter squared per mil filmthickness (gm/m² /mil), a film thickness of 5 microns was calculated.The coating was smooth and tack free with no noticeable pinholing orcratering. The coated circuit board was placed in a Kepro® ModelBTX-200A UV exposure frame with a polyester film photomask showing linesand spaces in 2 mil increments from 20 mils to 2 mils, and a "Stouffer21 step density Step Tab" (available from Kepro Circuit Systems, Inc.).The step tablet is a variable-density tablet based on a change inoptical density; it has been used to indicate the change in the level ofresist polymerization with exposure dose. Exposure time was threeminutes. Following exposure, the photo tool was removed easily from thecoated circuit board. No apparent residual coating material remained onthe photo tool.

The exposed coating was developed by immersing the panel in a developingsolution that was prepared in a essentially the same manner as disclosedin Example 1, at 80° F. for 75 seconds. The panel was then rinsed with awater spray and air dried.

Step 8 was held on the Stouffer 21 step - Step Tab Tester (availablefrom Kepro Circuit Systems, Inc.) and a resolution 2 mil lines andspaces was reproduced from the photo tool image. Etching occurred byspraying the sample with ferric chloride solution in a Kepro® ModelBTE-202 Bench Top Etcher (available from Kepro Circuit Systems, Inc.)Etchant temperature was 110° F. and exposure time was 140 seconds. Aresolution of 2 mil lines and spaces was achieved following etching.Stripping was accomplished by placing the etched panel in a bath of 10weight percent tactic acid at a temperature of 140° F. for 120 secondsfollowed by a water spray. Half of the panel was covered with tape andthe coated panel was again etched to check the efficiency of stripping.All of the copper was removed from the untaped coated panel surfacefollowing 120 seconds in 110° F. ferric chloride.

EXAMPLE 3A

The procedure of Example 2A was repeated except that 490 grams (1.49equivalents) of the urethane acrylate of Example 1A were used (insteadof 0.99 equivalents). A milky white dispersion with the followingproperties resulted: (1) Total solids at 110° C.: 34.4 percent; (2)Milliequivalent acid/gram: 0.129; (3) Milliequivalent base/gram: 0.265;(4) Particle size: 1250 angstroms (Å) and (5) GPC molecular weight:Mn=12,550; Mw=93,689; Mz=652,833.

EXAMPLE 3B

An electrophoretic bath was prepared by blending 619.7 grams of thedispersion of Example 3A with 21.8 grams of trimethylolpropanetriacrylate. A solution of 9.7 grams of 2-isopropyl thioxanthane and 9.7grams of hexyl cellosolve was added to the blend under mild agitation.The entire bath was reduced to 10 percent solids using deionized water.The resulting bath had a pH of 4.35 and a conductivity of 635 micromhosper centimeter.

Panel preparation, electrocoat bath set up, exposure time, developing,and etching conditions were the same as those for Example 2B. Coat outconditions were 85° F. bath temperature, 50 volts electrical potentialfor 30 seconds. A film build of 5.7 microns was obtained based on anassumed coating density of 30 gm/m² /mil. The coating was smooth andtack free with no noticeable pinholing or cratering. Following exposure,the photo tool peeled easily from the circuit board. No apparentresidual coating material remained on the photo tool. The photoresistwas developed with the aqueous acidic solution of this invention asfollows:

Following developing, a "step 10" was held on the Stouffer 21 Step Taband a resolution of 4 mil lines and spaces was reproduced from the phototool image. Some 2 mil lines were removed during developing. Followingetching, a resolution of 4 mils was achieved.

EXAMPLE 4C

Examples of epoxy-fiberglass circuit boards, (available from Nelco,Inc.), measuring 8 cm×12 cm with 1/2 ounce per square foot of copper onone side were electrophoretically coated with a cationic aminecontaining UV curable photoresist which was essentially the same asdescribed in the foregoing examples. Prior to coat out, the boards werecleaned in a detergent solution (ASTM D2248-65) at 165° F. for 5minutes. The panels were then washed with hot water followed by adeionized water rinse, and oven dried at 820° C. for 5 minutes.

These examples were submerged in the prepared electrophoretic bath (10%solids) along with a stainless steel anode. The sample (cathode) wasattached to a DC power supply and an electrical potential of 250 voltswas applied to the system for 30 seconds. The bath temperature was 95°F. Panels were removed from the bath, rinsed, and oven baked for 1minute at 120° C. to give a smooth, pinhole-free film of approximately10.4 mil dry film thickness.

Exposure occurred using a 5 kilowatt vacuum exposure frame from ORCManufacturing Co., Ltd. A phototool with a line-space resolution ofbetween 1 and 20 mils was placed on the top of each panel in directcontact with the photoresist layer. These were then exposed to a dosageof 150 millijoules per square centimeter (μj/cm²) (at the phototoolsurface).

50 grams of the developing solution of Example 1 was added to 1,950grams of deionized water. This bath was held at a constant temperatureof 84° F. during developing. An exposed sample was submerged in thedeveloping solution and waved slightly for 45 seconds. The panel wasremoved from the bath and rinsed with a water spray. Good resolution(non exposed areas clean to copper, no pick off of lines) to 1 mil wasobtained.

EXAMPLE 4D

70 grams of the developing solution of Example 1 was added to 1,930grams of deionized water. This bath was held at a constant temperatureof 84° F. during developing. An exposed panel was submerged in thedeveloping solution and waved slightly for 40 seconds. The panel showedslight underdevelopment (small photoresist specs remained in spaces) butotherwise there was good resolution and no pick off of lines down to 1mil.

EXAMPLE 4E

Epoxy-fiberglass circuit boards were prepared as they were fordeveloping experiments. A cationic, amine containing UV curablephotoresist was electrophoretically applied to the samples with anelectrical potential of 170 volts for 30 seconds. Bath temperature was95° F. Following rinsing and a 1 minute at 120° C. dry off, a film buildof 0.3 mils was obtained. Panels were exposed using a vacuum exposureframe and phototool identical to that used in the developmentexperiment. A dosage of 199 millijoules per square centimeter (μj/cm²)(at the phototool surface) was used for exposure.

Development occurred in a 2.5% developing solution of Example 1 at 80°F. Development time was 2 minutes, followed by a water spray. No etchingoccurred between developing and stripping.

EXAMPLE 4F

A stripping solution comprising a 4.0% solution of tactic acid was heldat a temperature of 120° F. Panels were immersed in the strippingsolution for 3 minutes. Samples were then removed from the strippingsolution and following a water spray, it was observed that all of theexposed material had been removed from the circuit traces. Panels wereimmersed in the stripping solution for 30 minutes. Following a waterspray, no exposed resist material remained on the circuit traces.

What is claimed is:
 1. An aqueous acidic developing solution for aphotoresist comprising an acid or mixture of acids each having fromabout 1 to 8 carbon atoms, in an effective combination with asolubilizing solvent for the photoresist.
 2. The aqueous developingsolution of claim 1 wherein the total acid concentration is from about0.25% to about 4% by weight based on total weight of aqueous developingsolution.
 3. The aqueous developing solution of claim 2 wherein thetotal acid concentration is from about 0.5% to about 1.5% by weightbased on total weight of aqueous developing solution.
 4. The developingsolution of claim 1 wherein the acid is selected from the groupconsistent of formic acid, acetic acid, hydroxy acetic acid, oxalic acidand di-lactic acid.
 5. The developing solution of claim 1 which containshydroxy acetic acid.
 6. The aqueous developing solution of claim 1wherein the solubilizing solvent for the photoresist is selected fromthe group consisting of diethylene glycol monobutyl ether, ethyleneglycol monobutyl ether, propylene glycol methyl ether, dipropyleneglycol methyl ether, propylene glycol t-butyl ether and ethylene glycolphenyl ether.
 7. The aqueous developing solution of claim 1 wherein thesolubilizing solvent is present in an amount of about 1% to about 5% byweight based on total weight of aqueous developing solution.
 8. Theaqueous developing solution of claim 7 wherein the solubilizing solventis present in an amount of about 1% to about 2.5% by weight based ontotal weight of aqueous developing solution.
 9. The aqueous acidicdeveloping solution of claim 1 comprising a mixture of at least twoacids, each having from about 1 to 3 carbon atoms, in combination with asolvent.
 10. The aqueous developing solution of claim 9 wherein thetotal acid concentration is from about 0.25% to about 4.0% by weightbased on total weight of aqueous developing solution.
 11. The aqueousdeveloping solution of claim 10 wherein the total acid concentration isfrom about 0.5 % to about 1.5% by weight based on total weight ofaqueous developing solution.
 12. The developing solution of claim 10wherein the mixture of acids is selected from the group consisting offormic acid, acetic acid, hydroxy acetic acid, oxalic acid and lacticacid.
 13. The aqueous developing solution of claim 10 wherein thesolubilizing solvent for the photoresist is selected from the groupconsisting of diethylene glycol monobutyl ether, ethylene glycolmonobutyl ether, propylene glycol methyl ether, dipropylene glycolmethyl ether, propylene glycol t-butyl ether and ethylene glycol phenylether.